Laser with hybrid-unstable ring resonator

ABSTRACT

A unidirectional ring laser oscillator has a travelling wave unstable mode in one transverse dimension and either a waveguide or freespace Gaussian mode in the orthogonal transverse dimension for coupling to large volumes of asymmetric cross section laser gain media. This device concept is shown to have unique and innovative features such as an exchange of left for right in the intracavity radiation profile without having to employ concave optics. Also, a high insensitivity to misalignment of one of the intracavity ring optics is achieved without having to suffer any deleterious effects associated with the high intensity of radiation normally encountered at an intracavity focal plane. Unidirectional operation of the laser is achieved using both intracavity and extracavity optical techniques.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT International ApplicationNo. PCT/RU2003/000220, filed on May 7, 2003, and published in English onNov. 18, 2004, as WO 2004/1 00328 A1, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the generation of single or low ordertransverse mode coherent light from large volumes of asymmetrictransverse cross section laser gain media to produce a filled-in,diffractively coupled output beam. This is accomplished by using ahybrid travelling wave unstable resonator. In general, it is well knownthat symmetric transverse cross section or asymmetric transverse crosssection standing wave unstable resonators have superior transverse modediscrimination characteristics compared with stable standing waveresonators. Utilization of hybrid unstable ring resonators is shown inthis disclosure to have features not obtainable using hybrid standingwave unstable optical cavities. For example, it is widely believed thathybrid negative branch standing wave unstable resonators uniquelyachieve their high level of insensitivity to end-mirror misalignmentbecause of the reversal of left-for-right that occurs at the focal planeof a concave mirror pair. Indeed, this is the reason that hybridnegative branch standing wave unstable cavities are said to be favoredover hybrid positive branch standing wave cavities in some laserapplications. However, this disclosure shows that the desirableintracavity left-for-right exchange feature and a low sensitivity tocavity misalignment can be achieved in a hybrid travelling wave cavitywithout having the deleterious characteristics associated with anintracavity focal plane. Likewise, because the cavity is travellingwave, spatial hole burning effects in some laser media are eliminated.Moreover, since the present invention is confined to a resonatorunstable in one plane only, does not employ a bifurcated waveguidegeometry, and is preferred to have all reflecting optics, the conceptdisclosed herein is scalable to very high average laser output powers.Accordingly, the general objects of the present invention are to providenovel and improved methods and apparatus of such character.

2. General Description of Unstable Resonator Prior Art

Since their first introduction [1] into the literature and their firstsystematic experimental and analytical investigation in 1965, unstableresonators have been applied to excimer, ionic, molecular, solid state,liquid state and free electronic laser media emitting over the spectralrange from the ultraviolet to the infrared. In this initial paper, modelosses were found by an ad hoc geometric optical analysis to beindependent of laser end mirror sizes and, while the cavity losses werefound experimentally to be expectedly large, the thought was expressedthat diffractive output coupling would be useful for transverse modecontrol. In 1967, a second paper [2] listed three general attributes ofunstable resonators: “1) Unstable resonators can have large mode volumeseven in very short resonators; 2) The unstable configuration is readilyadapted to adjustable diffraction output coupling and 3) The analysisindicates that unstable resonators should have very substantialdiscrimination against higher-order transverse modes”. The firstexperimental evidence of high transverse mode discrimination in a laserwith unstable resonator was reported in [3]. Over time, these threeattributes have been confirmed experimentally and theoretically manytimes. In 1967 and 1969, the concept of standing wave unstableresonators was expanded to a confocal concept when several innovativeunstable ring resonators were introduced for the first time [4] andbriefly discussed [5,6] with the comment expecting “a new possibility ofconstructing unidirectional ring generators” [5, p 1002]. Thesepublications [4-6] introduced unstable ring cavities for the first timewith and without intracavity focal regions. These ring geometries werefurther explored [5,6] and for the first time it was found that “eventhough the losses of modes propagating in opposite directions areidentical, their substantially different volumes in such cavities canobviously favor a unidirectional generation” [5, p 1002].

Although the CO₂ laser medium was widely appreciated as one of a numberof ideal candidates for the application of an unstable resonator system,it took nearly five years after the first introduction of unstableresonators [1] for details of such a resonator system to be reported in1969 [7]. This work reported the use of a positive branch, confocalunstable resonator to generate a maximum cw output power of 22 watts andemployed an annular coupler to generate a collimated fundamental outputmode in the form of a near field annulus. Within a year of the firstpublication detailing the true diffraction losses in 1970 [8] of up tothe first six lowest loss modes in a number of circular mirror, standingwave unstable resonator cavities, an unstable resonator system with a cwCO₂ output power of 30 kW was reported [9].

In 1972, a patent for a unique confocal ring unstable resonator wasfiled [10]. Also, about this time a number of experimental CO₂ laserstudies were published exploring the very detailed characteristics ofstanding wave confocal unstable resonators [11], unidirectionalsymmetric confocal ring resonators [12], asymmetric confocal ringunstable resonators [13] and injection locking and regenerativeamplification in standing wave and travelling wave unstable cavities[14]. Without exception, the experimental studies of the measureddiffraction losses in confocal standing wave unstable cavities [11] wasshown to be in complete agreement with the losses predicted by rigorousdiffraction theory [8]. This agreement includes the details of theresonator loss characteristics near the transition between the lowestloss symmetric mode and the next lowest loss symmetric mode [Ref. 11,FIG. 17]. Likewise, unidirectional operation in travelling wave unstableresonators, first proposed mid-1968 [4] was achieved [12,13] asinitially envisioned based only on the placement of the gain medium [5]within the unequal forward versus reverse mode volumes to favor one ofthe travelling wave directions. Moreover, the utility of travelling waveunstable resonators was shown to be a powerful resonator approach forapplying to the concept of laser regenerative amplification [14]. Inthis case, unidirectional operation is shown to be strongly enhanced bythe output mirror of the injection laser [Ref. 14, FIG. 35] whichfunctions as the reversing mirror [Ref. 10, FIG. 24, element 24].Unidirectional operation was shown to be easier to achieve in unstableoptical ring regenerative amplifiers as compared to standing waveunstable resonators because no isolator is required [Ref. 10, FIG. 29].

All told, within a decade of first being introduced and analyzed theunderstanding of unstable optical resonators proceeded from an initialgeometric optical approach [1] to a full iterative diffractive approach[8]. Along with this decade of theoretical work, CO₂ output powersincreased over the range from 20 W in an initial standing wave device[7] to eventually cw output levels speculated to be in the multi-hundredkW cw with an asymmetric ring unstable device design [12,13].

It is interesting to note historically that the original concept ofunstable optical resonator [1] proposed by Siegman in 1965 was neversubmitted to any patent office for patenting. Perhaps this was due to alack of a good diffractive analytical model for unstable resonators inthe early days of discussion and development. Meanwhile sufficientpractical utility of confocal unstable resonators was predicted in 1968and demonstrated experimentally in 1969 independently in [7] and [15].Due to these investigations the positive branch unstable confocal(telescopic) resonator innovation has been patented in Russia [16] withthe priority date 18.03.1968, but for a long time remained unknown tothe world laser community. The ring unstable resonator innovation [4],proposed in 1968 was not ever submitted to any patent office until 1972.Retrospectively, this may be due to general misunderstanding of howcompletely the reverse wave in the cavity can be suppressed. In 1972unstable ring laser resonators were patented in [10] due to developmentof efficient concepts of unidirectional operation of lasers with suchresonators. In any event, a contemporary review of unstable resonatorworks of that period can be found in [6,17] and a most thoroughdiscussion of all these and other types of unstable resonators alongwith detailed references in [18,19].

Stable ring resonators were well known in the laser art of the late1960's, having been introduced earlier for, among other things,applications requiring the sensing of physical rotation of objects in aninertial gravitational field [20]. For this application, the differencefrequency between the forward and reverse ring waves was found to beproportional to the angular rotation rate of the ring laser system.Unstable ring resonators are distinguished from stable ring resonatorsin that the mode diameters in the forward and reverse directions aregenerally different in unstable ring cavities but the same in stablerings. This is the basis of one of the ways unidirectional operation[5-6] can be achieved through the use of an intracavity aperture. Also,suppression of one of the oscillation directions in either symmetric[12] or [6,12,13] asymmetric unstable ring resonators can be achieved byjudicious placing of the intracavity gain medium. To accomplish this,one places the gain medium intracavity where the mode volume for one ofthe travelling waves is large and the other travelling waves is smaller[17, FIGS. 16,17]. In a near symmetric unstable ring cavity, the ratioof forward to reverse wave output power was measured to be nearly afactor of 20 [12, FIG. 6]. Another way of achieving unidirectionaloperation is through the use of a reversing mirror [10, FIG. 2] locatedoutside the cavity. Indeed, the aspect of unidirectionality in bothstable and unstable symmetric aperture resonators is central to thenotion of achieving regenerative amplification without the introductionof an optical isolator between the master oscillator and regenerativeamplifier [14, FIGS. 8, 29]. Likewise, in such applications as diverseplasma diagnostics [21] or analysis of laser spectral composition [22],ring geometries are highly advantageous and even essential. In all theseapplications, inventions or devices, universally and without exception,it should not be surprising to find that there is always some discussionof both directions of propagation in the ring geometry.

Obviously, in a travelling wave optical geometry, since the opposingdirections of propagation exit the optical device in distinct and uniquedirections, to discuss only one of the propagating directions isequivalent to discussing only half of the optical problem. Indeed,without such a discussion it is impossible to even know with certaintywhich of the two counter propagating modes is being used for output orwhich direction the output will be extracted. Conversely, absent such adiscussion of both propagation directions, such inventions or deviceshave to be considered to be fundamentally standing wave in nature andapplication.

Beyond the simple concept of directionality that one finds as the mostdistinguishing feature between stable ring resonators versus stablestanding wave resonators, the differences between unstable ringresonators and stable ring resonators is far richer and more complex.For example, in a stable ring resonator, the mode diameters of theforward and reverse waves at any location in the resonator and the totalmode volume of two waves is the same. In contrast, the mode diameters ofthe forward and reverse waves at any location in an unstable ringresonator and the total mode volumes of the two counter propagatingwaves are generally not the same.

For illustrative purposes, suppose an unstable ring resonator is bothconfocal and asymmetric. For this discussion, confocal refers to thefact that the design is such that either the forward or reverse wave isextracted from the resonator as a collimated output. Asymmetric in thiscase refers to the fact that the distance between the beam expansionoptics is greater (or less than) the remaining portion of the perimeter.For such an asymmetric confocal case [10], the resonator is confocal inonly one ring direction. Restated, “this kind of directional asymmetrycan only be accomplished in a [unstable] ring resonator” [19, p 839 line28,29]. Therefore to completely and unambiguously describe the modalproperties of unstable ring resonators, they have to be discussedentirely separately from stable standing wave, stable traveling waveresonators and also standing wave unstable resonators.

Consequently, with respect to inventions claiming novelty by employingvarious types of symmetric aperture or hybrid unstable resonators, suchinventions cannot be said to include unstable ring resonators unless thepatent itself specifically includes a discussion as to how one of theunstable ring mode directions will be effectively suppressed. Likewise,some discussion should be presented as to what the shape of theunsuppressed travelling mode will be relative to the laser gain mediumif it remains unsuppressed, since being unsuppressed, will represent adirection from which significant laser output power will be emitted. Inthis regard, U.S. Pat. No. 5,097,479 [23] conforms to this notion bydescribing the suppression of one of the travelling waves in a twomirror, bifurcated unstable ring resonator for application with a slabtype CO₂ laser medium. Likewise, U.S. Pat. No. 3,824,487 [10] conformsto this requirement since it discusses both the reverse wave and theaccommodation of the unsuppressed wave to the large volume of gainmedium. On the other hand, U.S. Pat. Nos. 4,719,639 [24] and 5,048,048[25] fail in this regard and thus their utility is fundamentallyself-limited to only hybrid standing wave unstable resonator geometries.

As herein disclosed, a laser with a travelling wave unstable resonatormode in one transverse dimension and either a waveguide or freespacegaussian mode in the orthogonal transverse dimension could be ideallysuited for effectively coupling to any type of gain media with anelongated transverse cross section. This, of course, assumes that one ofthe unstable ring oscillation directions can be effectively suppressed.If so, this invention can be advantageously applied to excimer, ionic,molecular, solid state, liquid state or free electron laser mediaemitting over the spectral range from the ultraviolet to the infrared.Such media might be pumped by an RF, dc, e-beam, incoherent light,coherent light or free electron source, or any combination of thesesources.

3. Description of RF Waveguide and Slab Laser Prior Art

While not limited thereto in its utility, the present invention isparticularly well suited for applications in high power CO or CO₂ laserswith rectangular discharge geometries. In general, a rectangulardischarge geometry is one wherein the transverse discharge cross sectionis elongated and the discharge is established most typically in eitherthe short transverse dimension (slab devices) or the long transversedischarge dimension (slice devices). A separate case exists for slicedevices where the discharge can be established perpendicular to theelongated transverse aperture. In all these cases, the ratio of the longtransverse dimension to the short transverse dimension is large and issuch that the long dimension is able to support a travelling waveunstable resonator mode in this long transverse dimension. Because ofthe elongated transverse cross section, such lasers can advantageouslyemploy optical resonators that have different functional andpropagational characteristics in the two different transversedimensions. For the first time an optical resonator of such a geometrywas experimentally investigated in [26] with a slab-type Nd-glass laser.The cavity comprised one planar and one convex cylindrical mirror suchthat the resonator was unstable along the longer transverse dimension ofthe slab (240 mm) and equivalent to a Fabry-Perot resonator in theshorter dimension (20 mm). Subsequently similar resonators were termedhybrid [27]. Thus two types of hybrid resonators to which the presentinvention is particularly applicable is one where the field in the shorttransverse dimension is described by either i) a waveguide mode or ii) afreespace gaussian mode while that in the long dimension is functionallydescribed by an unstable resonator mode.

In a preferred embodiment of this invention, the optical configurationdisclosed could find utility in high-power collision cooled waveguidegas lasers as disclosed in the “slab” discharge geometry of '639 [24]and in the “slice” discharge geometry as disclosed in '663 [28].

A slab waveguide laser excited by transverse high-frequency electricdischarge comprises a waveguide formed by the reflecting surfaces of twoelongated electrodes disposed parallel and in opposition to one another.The electrodes are made of a material highly reflective to laserradiation, thus ensuring low radiation losses in the waveguide. The gapbetween the electrodes is filled by a gas gain medium, which is excitedby a transverse electric discharge generated in the gas medium whenhigh-frequency pump power is applied to the electrodes. Mirrors makingup a standing wave laser resonator are disposed near both ends of thewaveguide formed by the elongated electrode surfaces. Besides excitingthe gas by the electric discharge and acting as the upper and lowerwalls for the optical waveguide, the electrodes play a role of coolingmembers and provide heat removal from the gain medium. To ensureadequate collisional heat removal from the discharge, the electrodes aremade of a material with a high thermal conductivity. In addition, thegap between the electrodes is made small and does not usually exceed afew mm. The electric field in such a discharge is directed essentiallyperpendicular to the cooling member surfaces and is oriented essentiallyalong the height of the gain medium cross-section. The typical examplesof such lasers are CO₂ [29,30], CO [31] and Xe [32] waveguide laserswith high-frequency excitation.

The “slice” laser discharge geometry disclosed in '663 [28] is alsocharacterized by a gas gain medium excited by a high-frequency electricdischarge and having an elongated transverse cross-section with ashorter and longer dimension. In distinction to the waveguide lasers thedischarge region is defined by having the discharge electric fieldestablished perpendicular to the short transverse dimension. In slicelasers, the discharge is confined between a pair of closely spacednon-conductive cooling members. These cooling members are disposed inopposition to one another such that the gap between their surfacesopposed to each other is not only small enough to provide collisioncooling of the gas filling the gap, but is suitable for guiding laserlight between the non-conductive surfaces. Thus the discharge in the gasis excited by a system of electrodes disposed such that the electricfield in the discharge chamber is essentially parallel to the surfacesof the cooling members, i.e. is directed transversely to the height(shorter dimension) of the gain medium cross-section. As disclosed in'663 [28] such “slice” lasers have a number of advantages in comparisonwith conventional waveguide lasers. These advantages include independentselection and optimization of discharge pressure and excitationfrequency, the possibility of combined use of RF and dc dischargeexcitation sources and a moderated effect of boundary layers near theelectrodes, among other virtues. Like slab devices, the slice dischargegeometry is relevant to CO, CO₂, Xe as well as other gas laser media.

Clearly, the small bore transverse RF discharge work with a microwaveand RF geometry reported in 1980 [33] and the stripline geometryreported in 1984 [34] both predate the filing of the slab geometry '639[24] in early 1987. On the basis of prior art, then, slab lasers werelimited for patent purposes to only waveguide operation in the smallerof the two transverse discharge dimensions and only unstable resonatoroperation in the larger dimension. In the slab waveguide geometry themodes of light propagating along the opposing electrode surfaces aredefined entirely by these surfaces and their mutual disposal. The slicegeometry '663 [28] and '256 [35], on the other hand, was an entirely newtransverse discharge arrangement when disclosed for the first time bypatent application. Therefore, the slice geometry is one that a muchwider set of hybrid resonator types can be applied. For example, withrespect to the slice geometries [32,35] the term “light guide” as usedin “slice” lasers has a broader sense than merely a waveguide mode. Thuslight propagation in the slice geometry can be either waveguide as in aslab device or a case where the intracavity radiation propagates withoutany interaction with the slice chamber sidewalls. One such case is wherethe intracavity mode in the small transverse slice dimension does nottouch the sidewalls and is best described as freespace gaussian mode infunctionality. Such a case takes place for instance in stable resonatorswhere the laser beam is confined by the resonator mirrors and does nottouch the light guide sidewalls because the sidewalls are not a boundarycondition for this type of intracavity modal propagation.

In the present disclosure the term “light guide” will be used in themore general sense taking in mind that it comprises all modes of lightpropagation from the waveguide mode to the free space propagation ofgaussian beams.

To increase the volume of the gain medium in a conventional symmetricaperture waveguide laser and the laser output power, while at the sametime maintaining a small electrode gap, a wide-aperture waveguide laserwith plane-parallel elongated electrodes of width considerably in excessof the electrode gap was proposed in U.S. Pat. No. 4,719,639 [24]. Theelectrode surfaces reflecting laser radiation form in this laser theupper and lower walls of an optical waveguide of a large width, whereinthe radiation propagating between the electrodes is confined by thiswaveguide only in the directions perpendicular to the electrodesurfaces. The waveguide is open in the directions parallel to theelectrode surfaces, and therefore the laser beam propagating along thewaveguide can expand in these directions in both opposite senses as infree space. A convex and a concave confocal mirror making up apositive-branch standing wave unstable resonator with a magnificationM>1 are disposed near the waveguide ends. In each transit from onemirror of this resonator to another and back, the laser beam expands byapproximately M times in the two opposite directions wherein the beam isnot confined by the electrode surfaces and can expand as in free space.To form only one output beam of a solid, i.e. filled-in cross section ina laser with such a resonator, the mirrors are usually disposed suchthat the axis of the unstable resonator formed by them is shifted topass near one of the open sides of the electrode gap. The output laserbeam is coupled out of the resonator from the other side of theelectrode gap, more particularly, near the edge of the convex mirroroverlapping only a part of the waveguide cross section. Such a “halved”configuration of the resonator allows formation at the laser exit, evenfor electrodes of a large width, of a beam of approximately rectangularsolid cross section with a close to diffraction-limited divergence ineach of the two transverse directions. The large electrode widthprovides excitation of a large gain medium volume and, as a result, ahigh output laser power.

It is known, however, that elongated discharge aperture lasers with apositive-branch unstable resonator having low magnification M are highlysensitive to resonator mirror misalignment, particularly to a change intheir angular position in the plane parallel to the electrodes [25], aswell as to wedge-type optical inhomogeneities in the same plane. Suchinhomogeneities usually form in the laser gain medium under dischargepumping. This was not unexpected given earlier published works [6,7]with conventional unstable resonators. In slab or slice CO₂ lasersmagnification M does not, as a rule, exceed 1.2 to 1.5. Therefore, ifspecial measures to increase the rigidity of the construction and toimprove the gain medium optical homogeneity are not taken, mirrormisalignment and gain medium inhomogeneities in such lasers result in asubstantial deformation of the radiation mode structure. This willresult in a sharp drop in output power, degradation of beam divergence,and an angular shift of the output beam, which cannot be tolerated inmost applications. The need for taking measures to solve these problemswill increase the laser cost.

It is well known to those of ordinary skill in the art of slab lasersthat a shift of the positive-branch unstable resonator axis to one ofthe electrode gap sides will result in transition of the resonatorconfiguration from a full resonator to the half configuration. However,it is not well appreciated that some intracavity radiation will stillescape from the resonator on the side of the electrode gap where theshifted axis is situated. Thus, while such a shift in the resonator axisallows the formation of essentially one output beam of a solid crosssection in place of two separate output beams, this advantageous featurecomes at the cost of forcing some radiation from the side of theresonator where it cannot be combined into the useful output beam. Thisentails a loss of radiation to the cavity thus reducing overall laserefficiency. Worse, such radiation can be inadvertently coupled back intothe desirable intracavity mode by stray reflections and force anundesirable higher order modes to compete for the full gain mediumvolume and thus the laser output.

To eliminate these difficulties, U.S. Pat. No. 5,048,048 [25] disclosedthe use of a negative-branch linear unstable resonator with amagnification M<−1 in the wide dimension of the discharge aperture. Thedisclosed confocal geometry is formed by two concave mirrors withdifferent radii of curvature having a common focal point inside theresonator. The confocal negative branch geometry can produce a onesided, filled in output beam as a result of the reversal of left forright which occurs at the confocal plane of the two concave resonatormirrors. In each pass through the focal point (beam focal waist), thelaser beam propagating along the waveguide between the mirrors of thenegative-branch unstable resonator becomes inverted in cross section, sothat after passing through the focal point the rays of the beam thatpropagated on one side of the resonator axis (which is a common normalto the surfaces of both mirrors) will emerge on the other side of thisaxis. Due to the laser beam rays passing alternately on one and theother sides of the resonator axis, the misalignments caused by theresonator mirror angular shifts in the plane parallel to the electrodesbecome efficiently compensated for |M| on the order of 1.2 to 1.5, asare compensated efficiently also the wedge-type optical inhomogeneitiesin the gain medium, which makes the resonator only weakly sensitive tosuch misalignments and optical inhomogeneities [6,7].

To provide one-sided coupling of the radiation out of thenegative-branch hybrid unstable resonator of '048[25], the size of oneof the resonator mirrors is chosen such that the distances from theresonator axis to the opposite edges of this mirror in the planeparallel to the electrode reflecting surfaces differ by more than |M|times. The other resonator mirror is chosen large enough that it doesnot constrain beam expansion in the waveguide. On the next pass throughthe resonator, the radiation propagating along the resonator on the sideof its axis opposite to the side on which the radiation is coupled outis reflected to the side where the beam exits, and is coupled out as theuseful output beam. As a result, the laser output radiation is a solidcross-section beam which, despite the beam expanding in the resonatorfreely in two opposite directions, exits it on one of its sides only.Thus, the presence of a focal waist in a negative-branch unstableresonator reduces the passive losses of the radiation generated in theresonator compared to the positive-branch halved unstable resonator, inwhich beam expansion in two opposite directions brings about inevitablypassive power losses from the resonator on its side opposite to the onewhere the useful output beam is coupled out.

However, because of the high local beam power density, the presence of afocal waist in the gain medium of a negative-branch unstable resonatormay give rise to undesirable nonlinear effects in the gain medium and togas breakdown, particularly in high-power pulsed lasers. Besides, theefficiency of use of the gain medium volume in such a resonator is lowerthan that in a positive-branch resonator because of the gain mediumbeing nonuniformly filled by the beam. Reducing the laser dimensions,which is usually achieved by folding the optical axis of the resonatorby means of an additional mirror, is also difficult in this arrangement,because the mirror placed into a negative-branch resonator to fold itsaxis will be too close to the focal waist to withstand the severeirradiation expected at high laser-power levels. Moreover, the mirrorsof a negative-branch unstable resonator should have a large curvature;indeed, their curvature radii should be on the order of the distancebetween the mirrors. As a result, to reduce the effect of the curvatureof these mirrors on field distribution over the waveguide height, i.e.,along the normals to the electrode surfaces, one has to use mirrors of acomplex shape, with different curvatures in the two mutuallyperpendicular directions, or to take special measures for wavefrontmatching, thus introducing additional losses in the resonator asdisclosed in U.S. Pat. No. 5,123,028 [36]. Besides, if the electrodewidth is increased noticeably, the increase in the width of thelarge-curvature mirrors is accompanied by a fast growth of sphericalaberrations entailing, in its turn, a substantial increase in the beamdivergence, which also places an obstacle on the way to usingnegative-branch unstable resonators in high-power waveguide lasers.Indeed, while FIG. 4 of '048 [25] shows that there is an advantage to anegative branch configuration over that of a positive branch design, thepower levels are noted to be relatively low. At the present time thereis some indication that at the several kW output power level in CO₂ slablasers there is some significant beam steering that may be caused by gasheating or other non-linear effects at the common focal region of theconfocal mirror pair.

U.S. Pat. No. 5,097,479 [23] proposed a wide-aperture waveguide gaslaser with a positive-branch ring unstable resonator completed withmeans for forcing unidirectional oscillation in the resonator [23, FIG.10, element 80]. This embodiment of a waveguide gas laser is pumped withhigh-frequency power applied between a pair of spaced electrodes. Thering resonator disclosed employs only two intracavity mirrors, that iswhy a complete ring resonator round trip must rely on a series ofdistributed reflections in the precisely curved bifurcated waveguidestructure. In this split bi-waveguide structure an unstable ring opticalresonator is formed with a closed axial contour to permit the extractionof an output beam with a solid cross section.

The ring resonator depicted in '479 [23] is formed by opticallycombining two precisely curved branches of two adjacent opticalwaveguide structures into a single optical unit. The precisely curved,bifurcated waveguide structure halves are coupled optically together bymeans of a pair of mirrors disposed at the ends so as to direct thelaser beam impinging on the mirror from one waveguide branch into theother waveguide branch. As a result, each of the mirrors turns the beamstriking it in the plane transverse to the electrode surfaces. Thus, apair of mirrors and two curved waveguide branches form in this laser aring resonator with a closed axial contour lying in the plane whichcrosses the electrode surfaces essentially at right angles and faceswith its opposite sides the open side ends of the waveguide. The ringresonator formed in this way in this plane provides a compact laserdesign, because the height of these waveguide branches is small comparedto their width and length. However, since it is impossible to form atravelling wave resonator of any kind using only two mirrors, the ringresonator optical circuit must rely on a proper, precise and equalcurvature of both waveguide branches. Clearly, the continuousreflections in the two curved bifurcated waveguide branches will addconsiderable intracavity optical loss and significant mechanicalcomplexity to the laser fabrication process as well. This is especiallyevident when it is remembered that one of the electrodes in a RF pumpedslab laser must be at an elevated RF potential. On balance, the apparentsimplicity of using only two intracavity ring mirrors must be weighedagainst the complexity arising from the requirement of achieving a verylow distributed optical reflection loss along both curved waveguidebranches. This low distributed loss must be achieved at the same time askeeping the elevated potential RF electrode from shorting out to thegrounded RF electrode. Moreover, to provide one-sided beam extractionfrom said positive-branch ring unstable resonator, the axial contour ofthis resonator is shifted such that it passes near one of the open sidesof the electrode gap. As can be seen from the optical diagram of '479[23], this hybrid ring resonator approach cannot be applied to a singleplane waveguide or a single plane guided wave structure since it onlyuses two intracavity optical elements. Furthermore, because of thebifurcated waveguide structure, the ring resonator cannot be madeasymmetric.

Because the traveling wave unstable resonator of '479[23] has no focalwaists inside the cavity, one-sided diffractive output extraction fromthe laser is provided only by shifting the axial contour to one of theopen sides of the electrode gap. Consequently, this laser suffers fromall the disadvantages discussed in relation to slab lasers with apositive branch, halved linear unstable system. Among thesedisadvantages are a high sensitivity to resonator mirror misalignmentand wedge-type inhomogeneities in the gain medium, as well as passiveradiation losses on the resonator side to which the resonator axialcontour is shifted occurring when extracting the useful radiation in theform of one beam. If anything, the bifurcated waveguide, traveling waveunstable resonator system of '479[23] adds, rather than eliminates,complexities of the other single slab hybrid resonator slab devices.

SUMMARY OF THE INVENTION

The principal object underlying the present invention, as applied tohigh-power, high-frequency excited, collision cooled gas lasers, is toprovide a hybrid unstable ring resonator system which has a lowsensitivity to laser resonator mirror misalignments and wedge-typeoptical inhomogeneities in the gain medium, as well as reduced passivepower losses in coupling out radiation from the resonator in the form ofone beam with a solid cross section.

This object is accomplished in that in a collision cooled gas laser withhigh-frequency-excitation, which comprises a pair of cooling members,each including an extended surface opposed such as to form a light guidefor propagation of optical radiation in the gap between said surfaces, alaser gas, disposed in said gap to generate laser radiation viaexcitation of said gas by an electrical discharge provided byhigh-frequency electric power supplied to the gas, and mirrors, forminga traveling wave ring resonator with a closed axial contour to generatein said light guide a laser beam, with said mirrors, in accordance withthe invention, being disposed such that said axial contour of saidresonator lies essentially in the plane, which is located between saidsurfaces forming said light guide for optical radiation and faces withits opposite sides said surfaces of cooling members, said traveling wavering resonator being unstable in said plane so that part of said beamexpanding in said resonator is coupled out of the laser as an outputbeam of solid cross-section, and the number of said mirrors and theircurvatures being such that any ray belonging to said beam andpropagating along said light guide inside said axial contour of theresonator emerges after a round trip to propagate outside of said axialcontour of the resonator, and any ray belonging to said beam andpropagating along said waveguide outside of said axial contour of theresonator emerges after a round trip to propagate inside said axialcontour.

The arrangement of the closed ring-resonator axial contour in a planelying between the surfaces of the cooling members means that each of themirrors of this ring resonator turns the beam reflected by it in saidplane. As a result of each such turn, the part of the beam thatpropagated along the light guide on the inner side of the closedresonator axial contour emerges on its outer side, and vice versa. Dueto such an inversion of the beam by each of the mirrors, chosen properlyin their number and curvature, any ray of the laser beam propagatingalong the light guide on the inner side of the resonator axial contourwill emerge on the outer side of the resonator axial contour after acomplete resonator round trip. Conversely, any ray of the laser beampropagating along the waveguide on the outer side of the resonator axialcontour will emerge on the inner side of the resonator axial contour,i.e., any ray passing at a distance from the resonator axial contourwill switch after a resonator round trip to the opposite position withrespect to said axial contour in the plane in which the beam expands asin free space.

Thus, the proposed traveling wave ring resonator provides passage of thelaser beam rays lying in said plane alternately on both sides of thering resonator axial contour. The location of the ring resonator axialcontour in the plane located between the surfaces of the coolingmembers, or essentially in this plane, is for positive-branch unstablering resonators the necessary condition for reducing the resonatorsensitivity to wedge-type perturbations in the resonator that areoriented in this plane. If the resonator axial contour lies in thetransverse plane, i.e., in the plane transverse to the cooling membersurfaces, as, for instance, in a laser with an unstable ring resonatorembodied in accordance with U.S. Pat. No. 5,097,479 [23], beamreflections from the resonator mirrors will not provide such aninversion of this beam in the plane of its free expansion, which isnecessary to reduce the resonator sensitivity to wedge-typeperturbations in this plane.

Due to the laser beam rays passing alternately on both sides of thering-resonator axial contour, a laser embodied in accordance with thisinvention provides compensation of the misalignments caused by angularshifts of the resonator mirrors in the plane lying between the coolingmembers, as well as compensation of wedge-type optical inhomogeneitiesin the gain medium, which are oriented in said plane. Therefore, theproposed laser is relatively insensitive to such resonator mirrormisalignments and optical inhomogeneities compared to known positivebranch hybrid laser designs, in which the axial contour of the ringunstable resonator lies in the plane perpendicular to the electrodesurfaces and, as a result, the sensitivity to wedge perturbationsoriented in the plane of free beam expansion is not reduced.

Besides, due to the laser beam rays passing alternately on both sides ofthe ring-resonator axial contour, the output beam of solid cross sectionin the proposed laser is coupled out only on one side of the axialcontour, despite the fact that the beam free expansion in the resonatoroccurs in both opposite transverse directions, in which it is notconfined by the electrode surfaces. For instance, the part of the laserbeam propagating along the inner resonator edge, i.e., on the inner sideof the axial contour, emerges after a complete round trip on the outerresonator side, where it can be coupled out as a useful output beam.Also, passive power losses on the inner side of the axial contour arepractically eliminated. Thus, according to this invention, passive powerlosses from the resonator are reduced compared to known positive-branchring unstable resonators, in which the axial contour lies in a planetransverse to the electrode surface and, as a result, beam expansion intwo opposite directions along the electrode surfaces brings aboutpassive power losses from the resonator near the edge which is oppositeto the one from which an output beam with a solid cross section iscoupled out.

In contrast to a prior art slab waveguide laser with a negative-branchlinear unstable resonator, using a positive branch travelling waveresonator as taught by the present invention having an axial ringcontour lying in the plane passing between the surfaces of the coolingmembers provides alternate passage of beam rays on both sides of theaxial contour. Significantly, this is accomplished without the need of afocal waist in the resonator. Because the laser embodied in accordancewith the present invention does not require the use of a focal waist,this laser will not suffer the undesirable nonlinear effects andbreakdown in the gain medium caused by the focal waist. Moreover, astaught by the present invention the gain medium is used in a volumetricefficient way due to the more uniform filling of the medium by theintracavity radiation flux. Reducing the laser dimensions by folding thelaser resonator optical axis is here also simplified. Besides, there isno need to use large-curvature spherical mirrors and take thecorresponding measures to reduce the deleterious effect of these mirrorson the field distribution over the waveguide height.

In the preferred embodiment of the proposed laser, the number of saidmirrors forming said traveling wave ring resonator is odd, and theircurvatures are such that the laser beam propagating in said waveguidedoes not have focal waists. If a laser beam does not have focal waists,in order for it to become inverted after a complete round trip, thenumber of the mirrors making up the ring resonator must be odd.

In accordance with the present invention, only one of said mirrorsforming said traveling wave ring resonator needs to be made convex toprovide the required magnification of the unstable resonator. In thisembodiment, to simplify the laser construction and cost the othermirrors making up the ring resonator can be planar.

In an alternate embodiment of the present invention, one of the mirrorsof the unstable ring resonator can be convex another can be concave withthe remainder being planar. Employing a ring unstable resonator with atleast one convex and one concave mirror can be seen to more easilyprovide for a desired curvature of wavefront at the laser output.

Compared to a slab laser made with the prior art technology of '479[23],in the preferred embodiment of the proposed laser, said surfaces of thecooling members can be essentially flat and disposed in parallel. Thisobviously simplifies the laser design and fabrication, as well aspermits one to raise its efficiency compared to the laser containingcurved waveguide branches. This is obvious because using curvedwaveguide branches in a laser can force the waveguide mode field toincrease toward the electrode facing the discharge gap with its concavesurface, as a result of which part of the excited gain medium locatednear the opposite electrode is used less efficiently.

In a preferred embodiment of the proposed invention, the distances fromthe edges of each mirror forming the resonator to the point ofintersection of the mirror surface with said axial contour are such thatthey provide coupling out of radiation from the resonator in the form ofa single beam, having a solid cross section and located in the vicinityof one of the edges of one of said mirrors, and confine in this wayexpansion of the beam in the resonator.

In other possible embodiments of the invention, however, said resonatorcan comprise a means for deflection of a fraction of radiation adjacentto the edge of the beam formed by resonator mirrors to provide couplingof said fraction out of the resonator and to confine in this way thebeam expansion in the resonator.

The preferred embodiment of the proposed laser includes means forproviding favorable conditions for propagation of radiation along thering resonator predominantly in one of the two possible oppositedirections. Such means permit one to provide essentially unidirectionallasing in the traveling wave laser resonator, thus offering apossibility of obtaining the maximum radiation power in a single compactoutput beam.

Said means providing favorable conditions for propagation of radiationpredominantly in one of said opposite directions can comprise a feedback(reversing) mirror disposed such that it does not affect essentially theradiation propagating in the resonator in the first of said twodirections, but reflects in the opposite direction at least a fractionof the radiation propagating in the resonator in the second directionopposite to said first direction, such that said fraction of theradiation is propagated in the resonator in the first direction.

In another possible embodiment of the invention, one of the resonatormirrors can have a hole, with its center disposed at the point ofintersection of said mirror with said axial contour, and said meansproviding favorable conditions for propagation of radiationpredominantly in the first of the two possible directions comprise afeedback mirror installed behind said hole such that it reflects in thefirst direction at least part of the radiation propagating through saidhole in the second direction such that at least part of the radiationpasses back through said hole and propagates along the resonator in thefirst direction.

Due to the feedback mirror, a coupling is established between the twocounter-propagating waves, which results in an additional amplificationof one of these waves. Mode competition setting in the gain mediumwithin the resonator in the stage of lasing creates favorable conditionsfor the beam to propagate predominantly in the first direction, so thatthe laser starts to operate essentially in a unidirectional travelingwave mode.

In yet another embodiment of the present invention, power from a lowerpower laser could be injected into a much higher power device so thatcoherent light from the low power device could control the wavelength ofthe high power device. In one arrangement the output from the lowerpower device could be injected into the reverse power direction at thehigh power output coupling mirror, or in another arrangement low powerradiation could be injected into the high power cavity via a hole in oneof the unstable ring resonator mirrors. In any of these embodiments ofthe present invention the lower power device would be directionallyisolated from any power extracted from the higher power device.

Parameters of the light guide in the embodiments generally discussedabove and consequently the mode of light propagation in the light guidecan be different. In some embodiments said traveling wave ring resonatorcan be stable in direction perpendicular to said plane of the resonatoraxial contour so that the beam is practically not guided by the coolingmember surfaces.

In some other embodiments said cooling members can be disposed such thatthey define a waveguide propagation mode of light in said light guide.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the drawings, in which

FIG. 1 is a perspective view of one of the variants of the embodiment ofthe collision cooled gas laser constructed in accordance with thisinvention;

FIG. 2 is a top view of a section through the electrode gap of thecollision cooled laser shown in FIG. 1;

FIG. 3 shows another variant of the embodiment of the collision cooledgas laser constructed in accordance with this invention;

FIG. 4 shows one more variant of the embodiment of the collision cooledgas laser constructed in accordance with this invention;

FIG. 5 a and FIG. 5 b illustrate one of possible schemes to provideunidirectional lasing in the collision cooled gas laser constructed inaccordance with this invention.

FIG. 6 shows one more variant of the embodiment of the collision cooledgas laser constructed in accordance with this invention and having 5mirrors.

To present the idea of the invention in a more revealing way, thedrawings are made not to scale.

DESCRIPTION OF THE DISCLOSED EMBODIMENTS

The collision cooled gas laser with high-frequency transverse excitationshown in FIG. 1 comprises a pair of cooling members 1 and 2 separated bygap 3 in which a gaseous gain medium (not shown) is disposed, forinstance, a typical gas medium of a waveguide CO₂ laser including CO₂,N₂, He, and other gases. In conventional slab waveguide lasers thecooling members simultaneously are used as electrodes. The discharge in“slice” lasers is excited by additional electrodes (not shown in theFigure).

The surfaces of cooling members 1 and 2 disposed in opposition arespaced close enough to form a light guide for propagation of opticalradiation. The electrodes are connected to a high-frequency generator(not shown) supplying high-frequency pump power to the gain medium.Cooling members 1 and 2 may also be accomplished by cooling means (notshown) to increase heat removal from the cooling members.

The outer contour of each of the cooling members 1 and 2 has triangularshape with truncated vertices, near which, in the immediate vicinity ofthe outer edge of gap 3, are disposed mirrors 4, 5, and 6 making up apositive-branch ring traveling wave unstable optical resonator with theclosed axial contour lying in the plane which passes between thesurfaces of cooling members 1 and 2 and faces with its opposite sidesthe surfaces of cooling members 1 and 2, respectively.

As shown in FIG. 2, to form a traveling wave ring optical resonator,mirrors 4, 5, and 6 are disposed relative to one another such that theyenclose a triangular axial contour (shown by a dash-dotted line). Thebisectrices of the angles in the corners of this triangular contour(shown by dashed lines) are simultaneously normals to the mirrorsurfaces such that the angle of incidence of an axial ray at the mirrorsurface is equal to the angle of reflection for every mirror. The axialcontour of the resonator shown in FIGS. 1 and 2 lies in the midplaneequidistant from the opposite surfaces of cooling members 1 and 2 makingup the light guide.

As can be seen from FIGS. 1 and 2, by arranging a closed ring-resonatoraxial contour in a single plane lying between the surfaces of thecooling members, one can provide, for equal areas of these members, amore compact design compared to prior art linear unstable resonatordesigns. Also, because laser beam propagation in a single plane lyingbetween the surfaces of cooling members 1 and 2 is not confined byanything, there is essentially no possibility of additionally turningthe radiation in this plane, and suffering the significant distributedreflection losses by the curved waveguide walls of prior art. Therefore,compared to the prior art as disclosed in '479 which employs twomirrors, two complicated and precisely curved waveguide planes anddistributed reflection along the four curved waveguide surfaces tocomplete one round trip through the ring cavity, the present inventionemploys three mirrors and only one simple axial resonator contour tocomplete a cavity round trip. By eliminating the distributed reflectionalong the two precisely curved waveguide surfaces and replacing it withreflection from a single intracavity optic, the present invention willhave significantly less intracavity loss than that of the two mirrorprior art ring resonator concept. Furthermore, even in a three mirrorring, the instant invention permits the beam expansion portion of theresonator to be comprised, for example, of mirrors 5 and 6 in FIG. 2.This permits the ring of the present invention to be made confocalasymmetric. It is not possible in the two mirror symmetric arrangementof the '479[23] prior art. Finally, the inversion of left for right inintracavity radiation in the present invention can be achieved withoutan intracavity focal region necessary in the prior art linear unstablecavities.

For the traveling wave ring optical resonator shown in FIG. 2 to be apositive-branch unstable ring resonator, mirror 4 in this variant ofembodiment of the invention is chosen convex, and mirrors 5 and 6 areplane. The convex mirror 4 may be, for instance, spherical with a radiusof curvature R chosen such as to provide an optimum magnification M ofthe unstable resonator. The magnification of the ring resonator shown inFIG. 2 (which is about that of the transverse beam dimension in thewaveguide midplane attained in a round trip of this ring resonator) isgiven, for a small enough angle of beam incidence on the convex mirror,by the expression

${M = \frac{\sqrt{{RL} + L^{2}} + L}{\sqrt{{RL} + L^{2}} - L}},$

where L is one half of the resonator axial contour length. Themagnification M of an unstable resonator is usually chosen such as toprovide an optimum efficiency of energy extraction from the laser gainmedium. The optimum value of M can be determined by the well-knownmethods employed to calculate the unstable resonator parameters inwaveguide gas lasers. For instance, for a typical collision cooledwaveguide CO₂ laser with an electrode gap of 2 mm, an axial contour 3.5m, long, and pump frequency of 81 MHz, the optimum value of M mayconstitute about 1.3-1.4.

The distances from the point of intersection of the resonator axialcontour with the surface of each of mirrors 4, 5, and 6, which make upthe resonator, to the edges or these mirrors are chosen such as toprovide coupling the radiation out of the resonator in the form of asingle beam of a solid cross section near an edge of one of the mirrors,e.g. edge 6 a of mirror 6, thus, to confine the beam expansion in theresonator.

For instance, the dimensions of mirrors 4, 5, and 6 along the normals tothe surfaces of the cooling members, exceed substantially the gap heightto prevent laser power leakage over the mirror edges. The size of mirror6 in the direction transverse to said normals is chosen such that thedistance measured in the waveguide midplane from the point ofintersection of the resonator axial contour with the surface of mirror 6to the first edge 6 a of this mirror is less than that to the secondedge 6 b of this mirror by not less than M times. The dimensions of theother mirrors, 4 and 5, in said transverse direction are large enoughthat the laser beam propagating in the resonator and incident on thesemirrors does not spill past their edges, i.e., that these edges do notconfine beam expansion in the resonator.

The laser shown in FIG. 1 includes also means providing propagation ofradiation predominantly in one of the two possible opposite resonatorround-trip directions, in this particular example, clockwise. For thispurpose, a small hole 8 (for CO₂ lasers the hole diameter may betypically 1 mm) is provided in mirror 4 on the resonator axis. Hole 8 isoriented such that the extension of the part of the resonator axispassing from mirror 5 to mirror 4, i.e., in the direction of thecounterclockwise resonator round trip, passes through this hole. A smallplane feedback mirror 9 intercepting said extension of the part of theresonator axis and oriented perpendicular to it is disposed behind saidhole 8 on the other side of the reflecting surface of mirror 4. Mirror 9is aligned such that the wave propagating counterclockwise along thering resonator and passing through hole 8 is reflected from mirror 9 andis propagated again through hole 8 back into the resonator, to becomeadded there with the wave propagating clockwise in the resonator.

The laser shown in FIGS. 1 and 2 comprises also a means for deflectingthe output laser beam in the desired direction, said means being formedby a plane mirror 10, as well as a concave mirror 11 serving to impartthe desired curvature to the beam wavefront.

The laser design shown in FIGS. 1 and 2 is meant only as illustration.In other variants of the invention embodiment, for instance, more thanone of the mirrors making up the ring resonator may be convex. One ormore of the resonator mirrors can be made concave to collimate theoutput beam or to impart the desired curvature to the beam wavefront.

The number of the mirrors making up the traveling wave ring resonator ina laser constructed in accordance with the present invention may be morethan three. For instance, in the waveguide gas laser shown schematicallyin FIG. 3 the number of the mirrors making up the ring resonator isfour, with mirrors 12 and 13 being concave, and mirrors 14 and 15,convex.

In the collision cooled gas laser presented schematically in FIG. 6 thenumber of the mirrors in the traveling wave ring resonator is five(mirrors 21 to 25). The mirror curvatures and their sizes are chosensuch that the ring cavity being unstable in the plane lying between thesurfaces of the cooling members has the required magnification M, andthe field distribution inside the resonator provides a good filling ofthe gain medium 26. In the variant shown in FIG. 6 mirrors 21 and 24 areconvex, mirrors 22, 23 are concave and mirror 25 is plane.

Note also that at least some of the mirrors making up the ring resonatormay constitute not single elements, as shown in the correspondingdrawings, but parts of a common mirror surface. Besides, in addition tomirrors, the ring resonator may include other known elements used toform such resonators.

Cooling members 1 and 2 must not necessarily be plane parallel. Forinstance, the surfaces of cooling members 1 and 2 may be curved withopposite signs of curvature in a plane transverse to the light guidewalls and passing through sections of the axial contour. The closedaxial contour of the resonator still lies in this case essentially inone plane, as in the above-considered variant of the inventionembodiment. In other variants of the embodiment, the distance betweenthe cooling members may be varied depending on the transversecoordinates in order to control the beam wavefront curvature in themidplane of the light guide symmetry.

It is obvious also that the axial contour of a traveling wave ringresonator may deviate within certain limits from the midplane passingbetween the surfaces of the cooling members, as long as this deviationdoes not degrade noticeably the laser output parameters.

The cooling members may also be made of separate sections. When thecooling members are used as electrodes the pump power can be suppliedseparately to each of them, as is well known in the art of RF excitedslab waveguide or slice guided wave lasers.

Coupling the laser power out of the resonator must not necessarily bedone near the edge of one of the mirrors, as shown in FIGS. 1 through 3.For instance, in the collision cooled gas laser constructed inaccordance with the principles of this invention and shown schematicallyin FIG. 4, the resonator is provided by a separate means to couple thebeam out of the resonator in the form of mirror 16, which is disposed onthe outer side of the resonator axial contour.

To provide unidirectional lasing in a traveling wave ring resonator,holes in resonator mirrors are not necessarily needed. For instance,feedback mirror 17 in a collisional cooled gas laser embodiment shownschematically in FIGS. 5 a and 5 b is disposed behind the resonatoroutput mirror, near its edge, in the shadow region for a wavepropagating clockwise in the resonator. At the same time, feedbackmirror 17 is aligned such as to reflect back the incident wavepropagating counterclockwise through the resonator. In this variant ofembodiment of the invention, convex mirror 4 serves as the output mirrornear whose edge the beam is coupled out of the resonator.

In the variant of the embodiment presented schematically in FIG. 6another possible design of the feedback is shown with an intermediatemirror 27 and a roof-reflector 28 in the feedback path. As shown in FIG.6, a polished lateral surface of mirror 21 is used as mirror 27 todirect back into the resonator the incident counterclockwise radiation20 after its reflection from properly disposed roof-reflector 28.

To operate the collision cooled traveling wave ring laser shown in FIGS.1 and 2, high-frequency pump power is supplied from an externalgenerator to excite the gas gain medium by electric discharge. In thecase of the conventional waveguide laser it is done via cooling members1 and 2 which are made in this case of a conductive material and play arole of electrodes. In order not to pump the area not filled by theintracavity radiation and, to increase the laser efficiency a depression7 in one of the electrodes can be made to prevent excitation of thedischarge in the area of the depression.

To operate the “slice” laser with the traveling wave ring resonator asshown in FIGS. 1 and 2 the high-frequency pump power (or its combinationwith dc power) is supplied to additional electrodes (not shown in theFigures). In both cases, when excited, the gain medium generates opticalradiation propagating in the waveguide (light guide) formed between theopposite surfaces of cooling members 1 and 2. The radiation, whosedirection of propagation coincides with that of the axial contour of thering resonator, is guided along this contour in a closed trajectory andis amplified in the gain medium, which results in self-excitation of thelaser beam near the resonator axial contour. When propagating along thering unstable resonator, the width of this laser beam increases in themidplane of the cooling members (in the plane of the drawing of FIG. 2)M times in each resonator round trip due to the curvature of the convexmirror 4, i.e., the beam expands as in free space. Beam expansion in thedirections transverse to the surfaces of cooling members 1 and 2 isconfined by these surfaces, thus forming the fundamental waveguide orGaussian mode depending on the light guide height.

Each of the three resonator mirrors, 4, 5, and 6, turns the laser beamcross section in the waveguide midplane passing between said electrodes.After each such turn, any ray of the beam reverses its position relativeto the resonator axial contour. No beam inversion occurs between mirrors4, 5, and 6, because this resonator does not contain concave mirrorsand, hence, there are no focal waists. Because the number of mirrors insuch a resonator is odd, any ray propagating in the resonator on oneside of the resonator axial contour emerges to propagate on the otherside of the resonator axial contour after a complete resonator roundtrip. To illustrate this situation, the ray in FIG. 2 passing clockwisealong the ring resonator through point A located on the inner side ofthe axial contour emerges, after a complete resonator round tripincluding consecutive reflection from mirrors 5, 6, and 4, at point Blocated on the outer side of the axial contour.

The width of a paraxial laser beam propagating along a ring unstableresonator expands in the midplane M times in each resonator round trip,until after some pass one of the beam edges spills past edge 6 a ofmirror 6, as shown in FIG. 2. However, the beam does not spill past theother edge 6 b of said mirror 6, because the distance from the axialcontour to edge 6 b is at least M times larger than that from the axialcontour to edge 6 a. After the next beam round trip along the resonatorand another increase of the width of the beam remaining in the resonatorby M times, the part of the laser beam that was near the second edge 6 bof mirror 6 emerges, due to the beam inversion in the ring resonatorconstructed in accordance with the invention, to propagate on the otherside of the axial contour, spills past the first edge 6 a of mirror 6and will be coupled out of the resonator. Thus, extraction of radiationfrom the resonator near the first edge 6 a of mirror 6 constrainsfurther beam expansion in both opposite transverse directions in whichit expands as in free space. At the same time, the edges of the othermirrors, 4 and 5, are far enough from the resonator axial contour thatthe beam propagating in the resonator does not spill past the edges ofthese mirrors, thus providing extraction of the radiation from theresonator in the form of one rather than two or more beams. The part ofthe broadened beam that spilled past edge 6 a of mirror 6 will leave thelaser after consecutive reflection from mirrors 10 and 11 in the form ofan output beam 18 with a close-to-rectangular solid cross section. Thecurvature of the concave mirror 11 provides collimation of the outputbeam in the transverse plane or, should this be needed, formation of aconverging out of the diverging beam exiting the ring unstableresonator.

Thus, in an embodiment of a laser made with the disclosed innovation,like that of FIGS. 1 and 2, the single side beam coupling from theresonator is effected due to the alternate passage of the rays making upthe laser beam on different sides of the resonator axial contour,similar to the way in which single side beam coupling in waveguidelasers with a negative-branch linear unstable resonator is achieved bypassing the rays alternately on both sides of the linear resonator axis.This linear negative branch arrangement permits the avoidance of passivepower losses from the resonator which occur in the positive-branchhalved unstable resonator with single side beam coupling. At the sametime, in contrast to the prior art negative-branch linear unstableresonators, the inversion of the beam cross section in the disclosedring resonator constructed in accordance with the invention, does notrequire a focal waist for this purpose and is attained due to the ringresonator axial contour being arranged in a plane passing between thesurfaces of cooling members 1 and 2, and to the properly chosen numberand curvatures of mirrors 4, 5, and 6 making up the ring resonator. Theabsence of beam focal waists in the resonator shown in FIGS. 1 and 2provides highly uniform beam intensity distribution and, hence, a highefficiency of use of the gain medium volume, and prevents undesirablenonlinear effects and breakdown in the gain medium.

Due to the rays forming the beam in a ring resonator constructedaccording to the present invention striking any of mirrors 4, 5, and 6alternately on both sides of the axial contour, the possible angulardeviations of these mirrors in the light guide midplane from theirrequired angular positions are canceled out similar to the way thisoccurs in negative-branch linear unstable resonators with a focal waist.Optical wedge inhomogeneities in the gain medium are canceled in asimilar way, because the positive path difference gained in one pass bythe outer part of the beam as a result of its propagation, for instance,over the “thick” side of such a wedge oriented in the light guidemidplane will be canceled in the next transit. This is a result of thesame part of the beam acquiring in the next transit a negative pathdifference, because the outer part of the beam will now be the innerpart and will propagate across the “thin” side of the same wedge. Thus,alternate passage of the rays making up a laser beam on opposite sidesof the ring resonator axial contour provides a substantial reduction ofthe sensitivity of a ring resonator constructed in accordance with theinvention to misalignments of mirrors 4 through 6 making up theresonator, and to wedge-type optical inhomogeneities in the gain medium,without any need of using focal waists in the resonator for thispurpose.

Thus, one embodiment of present invention suitable for a collisioncooled laser with a traveling wave ring resonator combines theadvantages of prior art linear positive ranch hybrid waveguide laserssuch as no intracavity focal region with the advantages of prior artlinear negative branch lasers such as low sensitivity to angularmisalignments, low sensitivity to wedge inhomogeneities and singlesided, filled-in output beam coupling.

As can be seen from the above discussion, for inversion in thetransverse cross section to be achieved, a laser constructed inaccordance with the teachings of this invention should preferably employan odd number of intracavity mirrors (3,5,7 etc.). With thisarrangement, the beam cross section becomes inverted in the longdimension without having to pass through an intracavity focal region.However, if a negative branch ring laser is constructed according withthe teachings of this invention to purposefully have an intracavityfocal region then in order to retain the inversion of the beam after acomplete round trip, the number of mirrors in the ring cavity must beeven. Alternatively, according to the teachings of this invention, onecould design a ring resonator to have an even number of intracavitymirrors. For such a situation, as shown in FIG. 3, if an inversion ofleft for right is deemed to be advantageous in a ring cavity with aneven number of mirrors, an intracavity focal region must be employed toprovide the reversal. For example, in FIG. 3, such waist 19, formed dueto the beam being focused by concave mirror 12, is disposed betweenmirrors 12 and 13. However, in contrast to lasers with negative-branchlinear unstable resonators, the waist region in a laser with a ringresonator can be removed from the pumped gain medium, and therefore thepresence of said waist will not degrade substantially the intracavitylaser parameters due to any deleterious non-linear effects.

In contrast to the cavity arrangement shown in FIG. 2, wherein beamexpansion in the resonator is confined by the resonator mirror 6, in thelaser shown schematically in FIG. 4 the beam is confined by anadditional mirror 16. In this case mirror 16 is disposed such that itdeflects and couples out of the resonator the part of the beam adjoiningthe edge of the laser beam formed by the optical resonator mirrors andlocated on the outer side of the axial resonator contour. Using thisseparate means to deflect the beam in order to couple it out of theresonator simplifies the alignment of the laser optical system andpermits one to easily control the output beam width by properly shiftingmirror 16 in the transverse direction.

The feedback mirror 9 shown in FIGS. 1 and 2 makes possible preferentialclockwise propagation of the beam in the resonator, thus providingessentially unidirectional lasing. Because of its small size, hole 8 inmirror 4 does not affect noticeably the wave guided clockwise along theresonator. The comparatively small part of the power carried by thiswave that passes through hole 8 is deflected by mirror 9 and does notreturn back into the resonator. At the same time, part of the wavepropagating counterclockwise in the resonator passes through hole 8 andis reflected by mirror 9 through hole 8 back into the resonator, topropagate there clockwise. Thus, a coupling is introduced between thetwo counter-propagating waves, which gives rise to an additionalamplification of one of these waves; in the example considered, the wavepropagating clockwise is amplified. Mode competition in the gain mediumforming in the stage of the onset of lasing in the resonator createsfavorable conditions for the clockwise propagating wave, and the laserstarts to operate essentially in a unidirectional mode. Such a reversingmirror is particularly effective with homogeneously broadened gain mediasuch as YAG or CO₂. As shown by calculations and experiments carried outby the authors of this invention, the presence of even a comparativelyweak coupling between counter-propagating waves in the resonator issufficient to provide essentially unidirectional laser operation.

In the cavity embodiment shown schematically in FIGS. 5 a and 5 b,unidirectional operation is achieved without using resonator mirrorholes, whose presence in waveguide lasers with a small electrode gap isnot always desirable. FIG. 5 a shows useful beam 18 propagatingclockwise in the resonator, and FIG. 5 b, the counterclockwise beam 20to be suppressed. Mirror 17, due to its being disposed behind the outputmirror 4, in the shadow region, does not introduce perturbations intothe clockwise-propagating beam 18. At the same time, thecounterclockwise beam 20 exits the resonator at an angle different fromthat of beam 18 and, as a result, becomes intercepted by mirror 17. Thebeam incident at mirror 17 is reflected by it in the opposite directionand is added to the beam propagating clockwise in the resonator. Thisresults, as in the laser shown in FIGS. 1 and 2, in creation offavorable conditions for the clockwise beam, and the laser startsoperating essentially in the unidirectional regime.

In the laser shown schematically in FIG. 6 another variant of thefeedback design is shown. Here the counterclockwise-propagating beamundergoes additional reflection from lateral side 27 of mirror 21 beforereflecting from feedback reflector 28. The angle between the surfaces ofmirrors 27 and 21 near their common edge 29 can be adapted to increasethe angle between the beams 20 and 18. The increase of this anglesimplifies interception of the counterclockwise-propagating beam 20 byfeedback reflector 28 and its reflection back into the ring resonator.The use of roof-reflector 28 instead of feedback mirror 17, decreasessignificantly the requirements to accuracy of the feedback reflectorangular positioning, provided edge 30 is properly positioned withrespect to edge 29.

In an initial embodiment of this invention, a cw output power of 350 Wwas extracted from CO₂ slab laser with a three mirror unstable ringresonator. The travelling wave unstable resonator components of thehybrid resonator consisted of two planar and one 60 m convex sphericalmaximum reflectivity intracavity mirrors to form a cavity having aperimeter of 2 L=1.3 m and a geometric magnification of 1.3. The 2 mmdischarge gap operated at a total pressure He—N₂—CO₂:1-1-6 equal to 70Torr. With a cavity configured similar to that of FIG. 5 a, the reversepower was measured to be suppressed to about a factor of 100 or 20 dBlower than that of the 350 W forward laser output. The one-sided,filled-in, asymmetric output beam with a cross-section of 2 mm by 12 mmwas measured to be very close to diffraction limited for each of theoutput dimensions.

The above-considered variants of a high-frequency-excited collisioncooled gas laser are presented only for illustration. The invention canbe embodied using any appropriate types of elements confining the gap,mirrors, gain media, pumping means and other components commonlyemployed in such devices. Those who are skilled in the laser art willrecognize that in place of mirrors, or alongside them, one can use anyequivalent optical means capable of providing the requiredtransformation of optical beams and change of their orientation.Therefore, while preferred embodiments have been shown and described,various modifications and substitutions may be made to these embodimentswithout departing from the spirit and scope of the invention.Accordingly, it is to be understood that the present invention has beendescribed by way of illustration and not by limitation.

REFERENCES CITED

[1] A. E. Siegman, “Unstable Optical Resonators for Laser Applications”,Proceedings of the IEEE, March 1965, pp 277-287.

[2] A. E. Siegman and R. Arrathoon, “Modes in Unstable OpticalResonators and Lens Waveguides”, IEEE, J. Quantum Electronics, vol.QE-3, 156-163, April 1967).

[3] Yu. A Anan'ev, N. A. Sventsitskaya, and V. E. Sherstobitov,“Transverse mode selection in a laser with convex mirrors”, Sov.Phys.-Doklady v. 13, p. 351-352 (October 1968) (In Russian: “DokladyAkademii Nauk SSSR”, v. 179, No 6, pp. 1304-1305, (July 1968), submitted22.05.1967).

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[6] Yu. A. Anan'ev, “Unstable resonators and their applications”, “Sov.J. Quant. Electron.”, v. 1, p. 565-586 (May-June 1972) (In Russian:“Kvantovaya Elektronika”, Ed. N. G. Basov, No 6, p. 3-28, 1971).

[7] W. F. Krupke and W. R. Sooy, “Properties of an Unstable ConfocalResonator CO₂ Laser System”, IEEE J. Quantum Electronics, vol. QE-5, pp575-586, December 1969.

[8] A. E. Siegman and H. Y. Miller, “Unstable Optical Resonator LossCalculations Using the Prony Method”, Applied Optics, Vol. 9, No. 12, p.2729-2736, December 1970.

[9] E. V. Locke, R. A. Hella, L. Westra and G. Zeiders, “Performance ofan Unstable Oscillator on a 30-kW CW Gas Dynamic Laser”, IEEE J. QuantumElectronics, vol QE-7, p. 581-583, December 1971.

[10] Carl. J. Buczek, Peter P. Chenausky and Robert J. Freiberg,“Unstable Ring Laser Resonators”, U.S. Pat. No. 3,824,487, filed 8 May1972

[11] R. J. Freiberg, P. P. Chenausky and C. J. Buczek, “An ExperimentalStudy of Unstable Confocal CO₂ Resonators”, IEEE J. Quantum Electronics,vol. QE-8, p. 882-892, December 1972.

[12] R. J. Freiberg, P. P. Chenausky and C. J. Buczek, “UnidirectionalUnstable Ring Lasers”, Appl. Optics, Vol. 12, No. 6, p. 1140-1144, June1973.

[13] R. J. Freiberg, P. P. Chenausky and C. J. Buczek, “UnstableAsymmetric Travelling Wave Resonators for High-Power Applications”, IEEEJ. Quantum Electronics, Vol. QE-9, p 716ff, June 1973.

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[16] Authors certificate SU 274254, filed 18.03.1968, Bulletin No 24,1970, p. 63. Authors Yu. A. Anan'ev, N. A. Sventsitskaya, V. E.Sherstobitov (In Russian);

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[21] Roger A. Haas, Peter P. Chenausky and Robert J. Freiberg, “LaserPlasma Diagnostic Using Ring Resonators, U.S. Pat. No. 3,885,874, Filed11 Jan. 1974, issued 27 May 1975.

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1. A collision cooled gas laser with high-frequency excitationcomprising: a pair of cooling members, each including an extendedsurface, opposed such as to form a light guide for propagation ofoptical radiation in the gap between said surfaces; a laser gas,disposed in said gap to generate laser radiation via excitation of saidgas by an electric discharge provided by high-frequency electric powersupplied to said gas, mirrors, forming a traveling wave ring resonatorwith a closed axial contour to generate a laser beam in aid light guide,wherein: said mirrors are disposed such that said axial contour of saidresonator lies essentially in the plane, which is located between saidsurfaces of said cooling members forming said light guide for opticalradiation and faces with its opposite sides said surfaces of saidcooling members, said traveling wave ring resonator being unstable insaid plane so that part of said beam expanding in said resonator iscoupled out of the laser as an output beam of solid cross-section, thenumber of said mirrors and their curvatures being such that any raybelonging to said beam and propagating along said light guide insidesaid axial contour of the resonator emerges after a round trip topropagate outside of said axial contour, and any ray belonging to saidbeam and propagating along said light guide outside of said axialcontour of the resonator emerges after a round trip to propagate insidesaid axial contour.
 2. The collision cooled gas laser as defined inclaim 1 wherein: the number of said mirrors forming said ring resonatoris odd and their curvatures are such that the laser beam propagating insaid light guide does not have focal waists within the resonator.
 3. Thecollision cooled gas laser as defined in claim 1 wherein: at least oneof said mirrors of the ring resonator is made convex, and the othermirrors are plane.
 4. The collision cooled gas laser as defined in claim1 wherein: said surface of said cooling members are essentially flat anddisposed in parallel.
 5. The collision cooled gas laser as defined inclaim 1 wherein: the distances from the edges of each mirror forming theresonator to the point of intersection of the mirror surface with saidaxial contour are such that they provide coupling out of radiation fromthe resonator in the form of a single beam, having a solid cross-sectionand located in the vicinity of one of the edges of one of said mirrors,and confine in this way expansion of the beam in the resonator.
 6. Thecollision cooled gas laser as defined in claim 1 wherein: said lasercomprises means for providing favorable conditions for propagation ofradiation along the ring resonator predominantly in one of the twopossible opposite directions.
 7. The collision cooled gas laser asdefined in claim 6 wherein: said means for providing favorableconditions for propagation of radiation predominantly in one of saidopposite directions comprise a feedback mirror disposed such that itdoes not affect essentially the radiation propagating in the resonatorin the first of said directions, but reflects in opposite direction atleast a fraction of the radiation propagating in the resonator in thesecond direction opposite to said first direction, such that saidfraction of the radiation is propagated in the resonator in the firstdirection.
 8. The collision cooled gas laser as defined in claim 6wherein: one of the resonator mirrors has a hole, with its centerdisposed at the point of intersection of said mirror with said axialcontour, and said means providing favorable conditions for propagationof radiation predominantly in the first of the two possible directionscomprise a feedback mirror installed behind said hole such that itreflects in the first direction at least part of the radiationpropagating through said hole in the second direction such that at leastpart of the radiation passes back through said hole and propagates alongthe resonator in the first direction.
 9. The collision cooled gas laseras defined in claim 1 wherein: said traveling wave ring resonator isstable in direction perpendicular to said plane of said axial contour.10. The collision cooled gas laser as defined in claim 1 wherein: saidsurfaces of said cooling members define a waveguide propagation mode oflight in said light guide.